A Trojan Horse is a type of malware that pretends to be something useful, helpful, or fun while actually causing harm or stealing data. Trojans are often silently downloading other malware (e.g. spyware, adware, ransomware) on an infected device as well. One of the most dangerous Trojans is Zeus.
Trojan viruses often are spread via an infected email attachment or a download that hides in free games, applications, movies or greeting cards.
Your computer will often slow down because of the burden on the processor. The best way to identify if you have been infected is to use a Trojan scanner, found in any Trojan removal software.
It's best to use a trojan remover that can detect and remove any Trojans on your device. The best, free Trojan remover is included in Avast Free Antivirus. When removing Trojans manually, be sure to remove any programs from your computer that are affiliated with the Trojan.